A little red can really make a room pop.

A brick red would even look good with the green. Choose some of the quirky shaped walls. But first choose bedding with red in it, to be sure the accent walls look good with it.

How about some red canvas bins for the cubbies instead? :)

Actually, I like the areas partim suggested above. Red wouldn't be my first choice for a bedroom wall colour (too energizing) but you could certainly find a couple of wall areas in this room for it if that's what he'd like. Beautiful dressers, btw.

Red is not a color condusive to sleep so I would use it in items that can be put away or changed easily as his favorite color changes. I do not love gray for a kids space but maybe a nice pale one would work since you have the angled ceiling I would only do one color everywhere.Then the next step is to get him to give up stuff he no longer plays with and take him shopping for bedding he likes. Try to make most storage closed and I always like a big cork board for saving pictures and other items that have meaning easy to remove the old and add the new as he ages.

Thanks Patricia! So are you suggesting even where the room currently has white paint, for example the bookcase, bench, stair railing I should now paint that the same color as the walls?

I like the idea of painting the whole room gray first, then step back and decide where to add the red. It's easy enough to change, so it's low risk if you both end up hating it! But, if it were me, I'd probably stick with gray walls and then do red accents with blankets, pillows, maybe a rug, and fun red "kid art" on the walls. Maybe he can make a canvas painting in red to hang on the wall? Could be an "abstract" finger paint or whatever the heck he wants - could be super fun to create something for his own room and add a big splash of red!